Potato, celeriac, carrot and swede gratin

Method

1. Use the slicing disc on your food processor to thinly slice the potatoes (you might have to halve them if they don’t fit into the feeder tube). Set aside in a bowl, cover with cold water and add a squeeze of lemon juice, to prevent discolouring.
2. Cut the celeriac and swede into pieces that will fit into the feeder tube, and thinly slice each as before. Set aside with the potatoes, making sure they’re covered in the acidulated water. Thinly slice the carrots.
3. Preheat the oven to 190°C/fan170°C/gas 5. Melt the butter in a small pan over a medium heat. Add the garlic and cook, stirring, for 1 minute. Stir in the thyme, then remove from the heat. Brush some of the thyme butter in a 1.8 litre ovenproof dish, about 6cm deep.
4. Drain the potatoes, celeriac and swede and pat dry with a clean tea towel. Arrange a third of the potatoes in a layer in the dish, slightly overlapping. Brush with more thyme butter, season, then layer up with celeriac, then another layer of potato. Butter, season and layer with carrot, more butter and then the swede. Press the layers down well, then brush with butter and finish with the remaining potatoes, overlapping to give a neat finish. Drizzle with any remaining butter.
5. Pour the stock into the dish around the edges, cover with foil and place on a baking sheet. Bake in the oven for 50 minutes, until tender.
6. Increase the oven to 200°C/fan180°C/gas 6. Uncover the gratin and bake for a further 30 minutes, until golden.